1. Field of the Invention
The present invention relates to a receiving circuit, provided in interference-suppressing communications system which comprises narrow-band conventional message modulation and additional pseudo-random phase shift keying (PN-PSK), in which the receiving circuit more specifically comprises a pseudo-random generator whose continuously repetitive pseudo-random sequence consists, in compliance with the transmittingside pseudo-random sequence, of an apparently random combination of the binary values L and H in a fixed clock pulse scheme and which actuates a phase keying which cancels the phase shift modulation produced at the transmitting side, and comprising a matched filter or correlation network, for example, a tapped delay device or a convolver for the purpose of producing a correlation pulse which always occurs when the pseudo-random sequence contained in the received signal reaches a specified location of the pseudo-random code, or when the correlation integral of the pseudo-random sequence contained in the received signal and of the pseudo-random sequence, produced at the receiving side, assumes a maximum.
2. Description of the Prior Art
Such interference-suppressing communications systems are described, for example, in the article of W. P. Baier: "Uberlegungen zu storsicheren drahtlosen Nachrichtenubertragungs-systemen" in the publication "Siemens Forschungs-und Entwicklungs-Berichte", 4, 1975, pp. 61-67. Such interference-suppressing communications systems are based on a conventional communications system which employs, for example, analog frequency modulation, digital frequency modulation, or digital phase modulation. Through additional frequency or phase shifts in the rhythm of a rapid binary pseudo-random sequence, a considerable spread of the bandwidth is effected. Such communications systems are designated as spread-spectrum communications systems.
The signal at the receiver input of spread-spectrum communications systems comprising pseudo-random phase hop modulation has the form ##EQU1## The quantities occurring in equation (1) have the following significance: A=Amplitude
p(t)=Binary pseudo-random sequence p(t) .epsilon.{-1, 1} with the clock pulse frequency f.sub.c PA1 t=Time PA1 .omega..sub.1 =Carrier angular frequency PA1 .phi.(t)=In comparison with the clock frequency f.sub.c narrow-band analog or digital phase modulation or frequency modulation (message modulation) PA1 s.sub.1 (t)=Received desired signal, and PA1 n.sub.1 (t)=Received interfering signal.
The receiver-side processing of spread-spectrum signals according to equation (1) can proceed by means of matched filters. However, the obstacle presented in such a case is that the received desired signal s.sub.1 (t) contains, in addition to the pseudo-random sequence p(t), known in the receiver, a message modulation .phi.(t) not known a prior i in the receiver, so that the design of an exact matched filter is not possible.
A possibility of keeping the interfering influence of the message modulation on the function of the matched filter, or correlator, respectively, low, resides in the employment of a sufficiently narrow-band message modulation .phi.(t). However, this restricts the oossibility to design an optimum system with regard to interference suppression in the message channel.
Matched filters for signals having pseudo-noise phase shift keying can be realized, for example, with tapped delay lines for acoustic surface waves; cf. the article of D. T. Bell et al, "Application of Acoustic Surface-Wave Technology to Spread Spectrum Communications", in the publication "IEEE Transactions on Microwave Theory and Techniques", Vol. MTT-21 (1973), pp. 263-271. In the interest of as great an interference suppression as possible, the tapped delay line should have as great an overall delay time T as possible and, hence, as many taps as possible. On the other hand, with the overall delay time T, the degradation, brought about by a message modulation .phi.(t) of a given bandwidth, increases. With the bandwidth B of the signal cos [.omega..sub.1 t+.phi.(t)], the maximum advantageous delay time is provided approximately by the relationship
ti T.congruent.1/B. (2)
The interfering influence of message modulation can therefore also be maintained low by virtue of the fact that delay devices, or convolvers, respectively, are selected with a sufficiently small delay time. However, one is thereby restricted in the possibility of a system design which is as interference resistant as possible with regard to the receiver synchronization. In the case of an increase of the parameter T substantially beyond 1/B, no further gain in signal-to-noise ratio is achieved. To the contrary, the result is again a reduction of the signal-to-noise ratio. Only in the case of B=0; i.e., in the case of .phi.(t)=0, would the signal-to-noise ratio improvement increase steadily with increasing T.
The restriction expressed by equation (2) would therefore be eliminated if no message modulation were contained in the received signal e(t); i.e. .phi.(t)=0. However, this is not realistic.